1. Field of the Invention
The present invention relates to a photodiode (PD) used in a system where light with a single wavelength is transmitted over a single optical fiber for bi-directional communications. More specifically, the invention relates to a photodiode used suitably for time-compression-multiplexing (TCM) transmission, or so-called "ping-pong transmission", based on time division using 1.3-.mu.m-region light for transmission and reception.
2. Description of the Background Art
FIG. 1 is a diagram showing a simplified constitution of bi-directional communications. A laser diode LD1 at a central office sends an optical signal. An optical coupler 2 introduces the signal into a fiber 3, and an optical coupler 4 guides it to a photodiode PD2 at a subscriber, such as a house. This is downstream traffic. A signal from the subscriber is sent out by an LD2. The signal travels through the optical coupler 4, the fiber 3, and the optical coupler 2 and reaches a PD1 at the office. This is upstream traffic.
There are two systems in bi-directional communications; one is simultaneous transmission and reception, and the other is alternate transmission and reception with time division. Some types use light having two or more wavelengths, and others use light having one wavelength. When light having two or more wavelengths are used, a wavelength-division-multiplexing coupler must be provided additionally. The present invention relates to the improvement of a photodiode used in a simple bi-directional communications system that transmits and receives signals having one wavelength on a time-division basis.
Since transmission and reception are conducted alternately, this system is called "ping-pong transmission." It is the simplest bi-directional communications since only one kind of light is used. Still it needs optical couplers in order to transmit the upstream and the downstream light over the same optical fiber. When two optical fibers are used, optical couplers are not necessary, but the cost for installing the fibers will increase. Therefore, transmission and reception over a single fiber is desirable. Optical couplers seem essential parts when coming and going signals travel over the same single fiber, because two kinds of light must be selected so that one kind is sent to the photodiode, and the other to the fiber after receiving it from the laser diode. However, optical couplers are expensive and increase the installation cost at subscribers. Therefore, optical communication without optical couplers is desirable.
A previous invention by the present inventors enabled the realization of this seemingly unattainable object. According to the invention, a photodiode is devised to absorb a half of light having a particular wavelength, and the photodiode and a laser diode are connected in tandem so that transmitted or received light propagates linearly. The laser diode is placed behind the photodiode so that the transmitted light from the laser diode passes through the photodiode and enters an optical fiber linearly. The photodiode absorbs half the transmitted light from the laser diode, allowing the other half to pass through. Similarly, the photodiode absorbs and detects half the received light from the fiber, allowing the other half to pass through and reach the laser diode, which is inactive then because of "ping-pong transmission" and causes no bad-effects. The condition of transmitted light and received light being halved is allowed when a sufficient amount of light is transmitted. This system does not require separation of optical path and eliminates an expensive optical coupler. In the system shown in FIG. 1 also, both the transmitted and received light are reduced to half by the optical couplers.
The previous invention described above was offered in Japanese patent application Tokuganhei 9-256107 filed on Sep. 3, 1997. This idea revolutionized the conventionally accepted knowledge that a photodiode is to absorb incident light 100 percent. Such a photodiode that absorbs a half of incident light is called a half-transmittance photodiode or simply transmittance photodiode.
FIG. 2 is a constitution diagram of a light transmission and reception module that has a combination of the above-mentioned transmittance photodiode and a laser. A fiber 62, a lens 126, a transmittance photodiode 64, and a laser 70 are aligned. No optical coupler is present. The light transmission and reception module may be produced at an impressively low cost. The ping-pong transmission with time division made this possible.
A half-transmittance photodiode has a thin absorption layer. A sufficiently thick (about 4 .mu.m) absorption layer of conventional photodiodes absorbs all the incident light. The half-transmittance photodiode is materialized by making its thickness "d" equal to (ln 2)/.alpha., where .alpha. means an absorption coefficient, and ln 2 is the natural logarithm of 2. Depending on a wavelength, the thickness is very thin, 0.7 or 1.0 .mu.m when InGaAs or InGaAsP are used for the absorption layer, for example.
The extremely thin absorption layer of a half-transmittance photodiode posed a new challenge. The present invention is directed toward further improvement of the half-transmittance photodiode. As applications of light communications spread, the use in a wider temperature range or with a lower source voltage is strongly demanded. Conventional indoor use allowed the device to operate within a range of 0 to 40.degree. C. The demand for outdoor use is widening the temperature range to -40 to +85.degree. C. It was difficult for the above-mentioned half-transmittance photodiode to operate stably in such a wide range of temperature. In addition, a source voltage of 5 V is being replaced by 3.3 V to preserve energy. With such a low voltage, p-n junctions sometimes do not receive sufficient voltage to operate normally.
Whereas a conventional photodiode having a thick absorption layer has a satisfactory temperature characteristic, the half-transmittance photodiode having a thin absorption layer developed by the present inventors showed a decrease in responsivity at low temperatures, a peculiar phenomenon not seen in conventional photodiodes.
FIG. 5 is a graph showing temperature characteristics of the above-mentioned half-transmittance photodiode with a parameter of source voltage. The axis of abscissa represents temperature (.degree. C.), and the axis of ordinate responsivity (A/W). The source voltages are 4 to 10 V, 3 V, 2 V, 1 V, and 0 V. When the source voltage is 4 to 10 V, the responsivity does not decrease, remaining about 0.47 A/W. However, when the source voltage is 3 V, the responsivity reduces by half at -40.degree. C. or below. When the voltage is 2 V, the responsivity begins to decrease at a higher temperature, 20.degree. C., and shows about 0.16 A/W at -20.degree. C. Such a remarkable decrease in responsivity at a temperature close to normal temperature is not seen in an ordinary photodiode, hence no such a decrease is reported.
This poor temperature characteristic deprives the invented half-transmittance photodiode of chances to be used outdoors or in cold climate. Because the photodiode is used in combination with an amplifier in an actual application, a source voltage of 3.3 V will drop to about 2 V across the photodiode. With such a low voltage, the temperature characteristic is too poor for the photodiode to be used below 25.degree. C.
A light transmission and reception module with a half-transmittance photodiode and without an optical coupler will lose the usefulness thereof significantly if the module must be replaced by a combination of a conventional, complete-absorption-type photodiode and an optical coupler for outdoor or cold climate use.
The principal object of the present invention is to offer a half-transmittance photodiode that does not become inferior in characteristics at low temperatures. Another object of the invention is to offer a half-transmittance photodiode that operates at low source voltages.
The principle of the photodiode of the invention is hard to understand because causes it is contrary to the common accepted knowledge for photodiodes. The invention is for fundamental improvement. This requires basic studies of the principle of photodiodes. An explanation of the internal structures of a conventional photodiode and the half-transmittance photodiode are given below, followed by a fundamental description of the meaning of "a p-n junction" and a"reverse bias".
A. Structure of a Conventional 100-percent Absorption-type Photodiode
A conventional InP-based photodiode has a stratified structure and impurity-density profile as shown in FIG. 3. In order to increase responsivity, incident light should be completely absorbed by a thick absorption layer (about 4 .mu.m). All the light having a shorter wavelength than a certain wavelength is absorbed by the absorption layer. A semiconductor and an insulation cannot absorb the light having lower energy (h.nu.) than the band-gap energy (Eg) (Eg&gt;h.nu.), because the light has insufficient energy (h.nu.) (h.nu.&lt;Eg) to excite electrons from the valence band to the conduction band. On the contrary, the light having higher energy (h.nu.) than the band-gap energy (Eg) (h.nu.&gt;Eg) is absorbed, because electrons are excited to the conduction band, generating free electron-hole pairs.
An absorption layer (Eg.sub.2) has a narrower band gap than that (Eg.sub.1) of the other layers, i.e., a substrate, buffer layer, and window layer (Eg.sub.2 &lt;Eg.sub.1). The light having energy higher than the band-gap energy of the absorption layer (Eg.sub.2) and lower than that of the other layers (Eg.sub.2 &lt;h.nu.&lt;Eg.sub.1) will transmit through the other layers freely, but will be absorbed by the absorption layer.
Hence, a photodiode for detecting 1.3 .mu.m or 1.55 .mu.m light has an InP substrate, InP buffer layer, InP window layer, and InGaAs absorption layer. The InP having a wide band gap will transmit all the light having a wavelength of 1.3 .mu.m or 1.55 .mu.m. The InGaAs having a narrow band gap will absorb and detect the light of 1.3 .mu.m or 1.55 .mu.m. As an absorption layer, quaternary InGaAs having a wider band gap is sometimes used. This material has a higher energy gap than the energy of 1.55 .mu.m light, so that it detects only 1.3 .mu.m light without reacting to 1.55 .mu.m light.
As mentioned above, the absorption layer having a narrow band gap converts light to free electron-hole pairs. The layer is also called an active layer because it can perform such energy conversion. When the layer is sufficiently thick, all the light having a shorter wavelength than that which corresponds to the band-gap energy of the layer may be absorbed with high responsivity. In order to increase the responsivity, it is necessary to thicken the layer as shown in FIG. 3, where the thickness of the InGaAs layer is 4 .mu.m.
Another essential element besides the absorption layer is a p-n junction. Without a p-n junction, a photodiode cannot be a photodiode. Traditionally, the p-n junction is provided in the absorption layer to separate the free electron-hole pairs by a reverse-bias voltage.
With an n-type semiconductor, a p-n junction may be produced by doping p-type impurities into n-type semiconductor layers (the window and absorption layers). Zinc, cadmium, or magnesium may be used as the p-type impurity, and it is common to produce a p region by diffusing zinc thermally. A plane where the donor density of an n-type semiconductor balances with the acceptor density of a p-type semiconductor is a p-n junction.
FIG. 3 shows the stratified structure and the diffused density of zinc for a standard (typically conventional) InP-based photodiode. The axis of abscissa represents the structure of the layers in the order of a window layer, absorption layer, buffer layer, and substrate. The axis of ordinate represents the diffused zinc density with a logarithmic scale. The n-type InP window layer has an impurity density of n=2.times.10.sup.15 cm.sup.-3, for instance. The n-type InGaAs absorption layer has an impurity density of n=1.times.10.sup.15 cm.sup.-3, for instance. Zinc diffuses thermally from the n-type InP window layer (from a to b) to the n-type InGaAs absorption layer (from b to c). Since the diffusion is conducted from the window layer, the zinc density decreases with increasing the distance from the window layer.
The region where p-type impurity density exceeds innate n-type impurity density becomes a p-region. Since the window layer has an n-type impurity density of n=2.times.10.sup.15 cm.sup.-3, zinc density must be higher than that. Actually, a much higher density (p&gt;1.times.10.sup.18 cm.sup.-3) is given to get lower resistance. This is a high-density p.sup.+ -region (A region). The boundary zone between the window and absorption layers is also a high-density p-type region.
The n-type InGaAs absorption layer is intrinsically low in impurity density as n=1.times.10.sup.15 cm.sup.-3 or so. Zinc density higher than that can change the layer into p-region (B-region: b to c). The zinc density decreases abruptly inside the absorption layer.
The density of the diffused zinc reaches the value p=1.times.10.sup.15 cm.sup.-3 at the point c, where the number of n-type impurities equals that of p-type impurities (n=p). This is a p-n junction, which is developed inside the absorption layer.
A region under the p-n junction is still n-type (n&gt;p). Zinc scarcely diffuses to the buffer layer and substrate. Since the absorption layer is low-density n-type, the region close to the p-n junction is low in p-type impurity density. In order to reduce the resistance by increasing electrical conductivity, a thick substrate has high-carrier density.
A conventional photodiode has an absorption layer as thick as 4 .mu.m. A p-n junction lies in the absorption layer. A depletion layer is in the absorption layer. A strong electric field exists only in the depletion layer.
When light with a long wavelength (h.nu.) enters a window layer (band gap is Eg.sub.1), if h.nu.&lt;Eg.sub.1, the light transmits through the window layer and reaches an absorption layer. If the energy of the incident light (h.nu.) is higher than the band-gap energy of the absorption layer (Eg.sub.2)(Eg.sub.1 &gt;h.nu.&gt;Eg.sub.2), the light generates free electron-hole pairs there. Newly generated free electrons are attracted toward the n-side by a strong electric field, and holes toward the p-side, so that they are separated and cannot recombine. Respective flows of electrons toward the n-side region and holes toward the p-side region generate an electric current between the electrodes. This current is known as a photocurrent. This phenomenon makes it possible to convert light into an electric current to be detected.
A source voltage generates an electric field in the depletion layers produced on both sides of the p-n junction. The reason why the p-n junction lies in the absorption layer is that a strong electric field must be produced in the absorption layer so that the generated free electrons and holes are separated. Therefore, it is a prerequisite for the p-n junction to lie in the absorption layer.
B. Structure of the Half-transmittance Photodiode Previously Offered by the Present Inventors
FIG. 4 is an example of the diffused-zinc density in the half-transmittance photodiode the present inventors offered before. An absorption layer is of In--GaAs having a narrow band gap. When thickness d of InGaAs is made equal to (ln2)/.alpha., where ln is the natural logarithm, and .alpha. is the absorption coefficient of InGaAs, a half of incident light is absorbed and the other half transmits. Because .alpha.=0.99 .mu.m.sup.-1 for 1.3 .mu.m in wavelength, the photodiode has a thickness of d=0.7 .mu.m, as thin as about 1/6 that of conventional photodiodes.
In order to form a low-resistance p-type layer in an n-type InP window layer, the density of the diffused zinc should be considerably high (p&gt;1.times.10.sup.18 cm.sup.-3). Since the InGaAs absorption layer is thin (0.7 .mu.m), very shallow diffusion is required to provide a p-n junction (n=p) in the layer. However, diffusion cannot provide a sharp edge of distribution thereof. If the p-n junction is planned to lie at the innermost part (j) of the absorption layer, the hetero-interface (i) between the window and absorption layers may have a zinc density of less than 1.times.10.sup.18 cm.sup.-3 as shown in FIG. 4. Because the p-region at the hetero-interface is a weak p-region, a potential barrier in the valence band tends to appear at the hole side.
A half-transmittance photodiode thus fabricated has a poor temperature characteristic as shown in FIG. 5. A responsivity drop at low temperatures is notable. A low source voltage particularly exacerbates this tendency. This phenomenon does not appear in conventional photodiodes. In order to study the reason why, the responsivity is plotted as a function of the reciprocal of temperature as shown in FIG. 6. The axis of abscissa is 1000/T (K.sup.-1); the axis of ordinate is responsivity with a logarithmic scale. The source voltage is 0, 1, 2, 3, 4, and 5 V. The same data used in FIG. 5 is used again with the axis of abscissa converted from temperature in Celsius to the reciprocal of absolute temperature. When a source voltage is 0, 1, 2, or 3 V, a liner line is produced. The gradient of these lines gives activation energy .DELTA.E.sub.a of 0.29 eV. When the temperature dependency of the intensity of a phenomenon may be written as exp(-.DELTA.E.sub.a /kT), where k signifies the Boltsman's constant and T an absolute temperature, the value over kT is called activation energy. When the temperature dependency of the probability of the occurrence of a reaction that requires to surmount a potential barrier of .DELTA. may be expressed in this form, Arrhenius originally called the energy activation energy.
In other words, FIG. 6 means that the phenomenon occurring in the photodiode having a thin absorption layer is performed by surmounting the potential barrier of 0.29 eV.
Why such a potential barrier is produced and where a potential discontinuity appears is explained below.
The reason why the half-transmittance photodiode has the conspicuous temperature dependency was thoroughly examined from various standpoints by the present inventors. The temperature dependency reveals that the phenomenon accompanies the activation energy of 0.29 eV. In searching for an explanation of this, it was noticed that 0.29 eV is close to half the difference between the band gap of InP (1.35 eV) and that of InGaAs (0.75 eV). It was inferred that the foregoing energy difference actually appears at the hetero-interface between InP and InGaAs. In the fabrication of the aforementioned half-transmittance photodiode, emphasis was placed on making the absorption layer thin and forming a p-n junction in the InGaAs absorption layer. The p-n junction is a plane that links the points where n-type impurity density is equal to p-type impurity density (p=n), and the InGaAs absorption layer is intrinsically low in impurity density.
Accordingly, the p-type impurity density must be reduced in the thin absorption layer (about 0.7 .mu.m). Because zinc diffuses thermally, the density gradient cannot be sharp depthwise. This means that the zinc density cannot be increased at the interface between the InP window layer and the InGaAs absorption layer (hetero-interface); i.e., the zinc density at the hetero-interface is low. It is considered that since the zinc density is low, charge carriers might be trapped at a potential barrier produced at the hetero-interface between the window and absorption layers. The height of the barrier is conjectured as (1.35-0.75)/2=0.3 eV or so. The necessary energy for the carrier to surmount the potential barrier at the hetero-interface is about 0.3 eV. The above-mentioned activation energy (0.29 eV) derived from the responsivity-temperature characteristic is almost equal to the energy needed for surmounting the barrier at the interface between the window and absorption layers. At low temperatures, as the disturbance by thermal motion decreases, the probability of surmounting the potential barrier decreases. This is considered to be the reason why the responsivity decreases at low source voltages and low temperatures.
A zinc-density profile was examined in an actual wafer sample depthwise by the secondary ion microspectroscopy (SIMS). The result for a conventional photodiode having a thick absorption layer is shown in FIG. 3. The results of the half-transmittance photodiode having a thin absorption layer previously offered by the present inventors are shown in FIG. 4. The conventional diode has a zinc density of more than 1.times.10.sup.18 cm.sup.-3 at the hetero-interface between the window layer (n=1.times.10.sup.18 cm.sup.-3) and the absorption layer (n=1.times.10.sup.15 cm.sup.-3) as shown in FIG. 3. On the other hand, the half-transmittance diode has a considerably lower zinc density than 1.times.10.sup.18 cm.sup.-3 at the hetero-interface; that is, a highly dense p.sup.+ -region does not exist at the hetero-interface between the window and absorption layers.
More specifically, zinc diffusion does not produce such a uniform high-density p.sup.+ -region as often explained in text books. As shown in FIG. 3, behind a high-density A region, there exists a B region where the density steeply declines. Further behind that, there is a C region where the density moderately lessens. A p-n junction (n=p) is developed in the low-density C region. On the other hand, the heterojunction produces a band gap difference, which confronts carriers as a barrier. If the carrier density is low or the temperature is low, the carrier cannot surmount the barrier. In conclusion, it is necessary to skillfully arrange the A, B, and C regions along with the position of the hetero-interface in order to operate the photodiode properly. If a p-n junction is developed in the thin absorption layer, the energy barrier at the hetero-interface interferes without exception.